minimum insulation resistance Mohms

[swifters]

Hi, I'm studying for my 2391 and tlc website gives the table below. 17th regs book gives 1 MegaOhm for 500v so I take it TLC are basing this on older regs book? Thanks Jon

http://www.bathroom-london.co.uk

Extra-low voltage circuits supplied from a safety transformer

250​

0.25​

Up to 500 V except for above

500​

0.5​

Above 500 V up to 1000 V

1000​

1.0​

 

[IQ Electrical]

The relevant bit is hidden at the bottom of Table 41.1 (note 2) in BS7671:2008.

 

[IQ Electrical]

Just one last question, why do you have to multiply the highest line to neutral value by 2 to obtain 3 phase prospective short circuit current?

Well if you go back to AC theory, you would in fact multiply by 1.732 to obtain a 3-phase PFC but the X2 rule of thumb builds in an error on the side of caution.

Of course if your instrument is capable, you can measure line to line fault current.

 

[swifters]

The relevant bit is hidden at the bottom of Table 41.1 (note 2) in BS7671:2008.

Where compliance with this regulation is provided by and RCD, the disconnection times in accordance with table 41.1 relate to prospective residual fault currents significantly higher than the rated residual operating current of the RCD (typically 2 I^n).

I can read it but I don't necessarily understand it...

 

[IQ Electrical]

Basically, it's saying that the disconnection times are based on fault currents far higher than the I delta n ratings of the RCD's that we traditionally test them against.

I think Appendix 3 has disconnection times for BS61008/61009 devices at 2X I delta n values (a test requirement for our continental friends).

 

[malcolmsanford]

Take a glance at appendix 3 in the BRB table 3A. You will see it gives you the value of 2 x delta N as a maximum of 150mS so your going to be well within that 200mS threshold.